Frequency measuring devices have wide use in industrial applications, both as part of direct control devices for rotating equipment and as components in various measuring devices. With the wide spread availability of digital processing equipment, it is common to convert many analog signals into digital signals for data processing. In one conventional transformation, analog input is converted to a chain of pulses which are produced at a rate which is functionally related to the analog input. The frequency measuring circuit must then convert this pulse rate into a digital signal for processing.
Prior to digital processing, it was common to measure unknown frequencies where the wave forms were generally sinusoidal. Under these conditions, known frequencies with a sinusoidal wave form were "beat" against the unknown frequencies, wherein a difference frequency would result. The prior art devices of this type are exemplified by U.S. Pat. No. 1,919,803 to Roetken and U.S. Pat. No. 2,131,559 to Granger. A plurality of discrete frequencies in each frequency range was generally beat against the unknown frequency until a second unknown frequency in a preselected frequency range was obtained. A second set of known frequencies was then beat against this second unknown frequency, and so forth. By adding the known frequencies which produced the beat frequencies within the preselected ranges, an approximation of the unknown frequency could be obtained. These prior art devices required manual manipulation to obtain the desired beat frequencies and were not readily adaptable to measuring a continuously varying frequency.
A second technique is illustrated by U.S. Pat. No. 2,576,900 to Brockman, which employed a circuit to count the incoming waves for a known period of time. Brockman illustrates a technique where the counting period is preset by setting a time interval determined by a selected number of waves from the known input, for example 1 .times. 10.sup.6 waves, so that the output would be a direct measurement of the unknown frequency. This technique is illustrated for pulse-type waves by U.S. Pat. No. 3,808,407 to Ratz, where either the known or unknown pulses are used to determine the counting interval. In Ratz, the known and unknown pulses are each counted until one reaches a preselected count and a second counting stage is then actuated until the second count reaches the same preselected total. In this fashion, the second counting stage directly displays the difference in the two pulse rates.
Yet another technique for pulse rate measurement is depicted in U.S. Pat. No. 2,913,664 to Wang, where a known and unknown frequency are combined to provide reset pulses to a scaler, such that an output pulse rate is produced which is functionally related to the unknown frequency and to some integer times a known frequency. Wang teaches that this output pulse rate is then converted to an analog output for visual display purposes. U.S. Pat. No. 2,985,773 to Dobbie provides yet another circuit for producing a difference frequency between a known and unknown pulse rate where the frequency difference appears on one output line if the known pulse rate is greater than the unknown pulse rate and the difference frequency appears on another output line if the unknown pulse rate is greater than the known pulse rate. The specific circuitry taught by Dobbie is composed entirely of NOR gates producing the desired logic. Dobbie does not teach any specific use for the outputs depicted therein, but teaches only a single logical network for obtaining the differential frequency outputs.
It is readily apparent that the above prior art devices are predicated on counting the unknown pulse or wave rate to obtain an indication of the unknown frequency. Thus, the output indication from the circuits is not continuous but only indicates the unknown frequency or pulse rate as the average which occurs during the counting cycle. It would be highly advantageous to provide an output which is a continuous representation of the unknown input frequency or pulse rate and to provide this output in a direct digital form for data processing or for direct conversion into a numerical display through standard binary decoding and display circuitry.
The disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus are provided for obtaining a continuous digital output which is functionally related to an unknown input frequency or pulse rate.